Electric pulse translator circuit



Feb, 13, 1951 A. H. REEVES 5 L ELECTRIC PULSE TRANSLATOR CIRCUIT FiledApril 1.7, 1948 2 Sheets-Sheet l Inventor ALfC HARLEY FEET 5 A tlorneyF@m 13, 19 51 A. H. REEVES 5 ELECTRIC PULSE TRANSLATOR CIRCUIT FiledApril 17, 1948 2 Sheets-Sheet 2 DELA Y NETWQQH lnucnlor ALEC HARLEYIQEEVES Altar/1e) Patented Feb. 13, 1951 ELECTRIC PULSE TRAN SLATORCIRCUIT Alec Harley Reeves, London.

England, asslgnor to International Standard Electric Corporation, NewYork, N. Y., a corporation of Delaware Application April 17, 1948,Serial No. 21,695 In Great Britain June 5, 19-17 8 Claims. 1

The present invention relates to electric pulse modulating anddemodulating arrangements employing cold cathode gas-filled dischargetubes.

When high signal-to-noise ratio have to be provided in pulse modulatorsand demodulators, the use of a gas-filled tube sometimes causes troubleon account of the slight irregularities in the moments of firing of thetube, which increase the noise introduced by the modulating anddemodulating arrangements.

It is the principal object of the present invention to overcome thisdifilculty, so that the advantages of using gas-filled tubes in sucharrangements may be secured while still conforming with the moststringent noise requirements.

This object is achieved according to the invention by providing anelectric pulse channel selector for a multichannel electric pulsecommunication system having n channels, comprising a cold cathode gasfilled electric discharge tube having a sequence of n discharge gapsincluding an output gap, arranged as a counter tube, means for applyinga train of input pulses to the tube in such manner as to fire the gapsin turn, means for deriving from the output gap a pulse corresponding toeach nth input pulse, and means for applying the output pulses as gatingpulses to render operative the equipment corresponding to a specifiedchannel.

The invention will be described with reference to the accompanyingdrawings in which:

Fig. 1 shows a schematic circuit diagram of a pulse modulator accordingto the invention;

Fig. 2 shows curves used in explaining the action of the modulator;

Fig. 3 shows a schematic circuit diagram of a pulse demodulatoraccording to the invention; and

Fig. 4 shows curves used in explaining the action of the demodulator.

Referring to Fig. 1, the discharge tube I preferably contains a gasfilling of 92% neon, 1% argon and 7% hydrogen at a total pressure of 100mm./Hg. It includes an anode 2 in the form of a flat plate or wire, acorrugated counting cathode 3 arranged parallel to the anode, and anoutput plate cathode 4. The tips of the corrugations form with the anodea sequence of discharge gaps, all of which are equal, except the one atthe left hand end, which is smaller; and

the gap between the cathode 4 and the anode 2 should be equal to thelarger gaps. Other forms of cathode could provide the desired sequenceof 2 gaps; for example a helical wire, or a metal comb with the teethprojecting towards the anode.

It will be assumed that the circuit of Fig. 1 is one of the channelmodulators in a six channel pulse communication system. In that case,the corrugations of the cathode 3 should provide five discharge gaps; ifthere are 12 channels, the cathode should provide n-1 gaps.

The cathode 3 is connected directly to ground, and the cathode 4 isconnected to ground through a network consisting of a condenser 5shunted by a resistor B. The anode 2 is connected through a loadresistor l to the positive terminal of the operating battery 8, thenegative terminal of which is grounded.

A train of regularly repeated rectangular pulses as shown in Fig. 2a isapplied from a local source (not shown) to terminal 9 which is connectedthrough a blocking condenser ill to the anode 2. The battery potentialshould be such that it is insufficient to start any discharge in thetube, but can maintain a discharge when one has been started by othermeans. The first of the pulses shown inFig. 2a then fires the small gapat the left hand end of the cathode 3, and the discharge is maintainedby the battery after the disappearance of the pulse. The spread ofionisation from the fired gap then primes the next one so that it isfired by the next pulse, and so on, until the sixth pulse fires the gapbetween the cathode 6 and the anode 2. The discharge spreads over thesmooth plate 4 so rapidly that a squeg is produced in conjunction withthe elements: 5, 6 and 1, which extinguishes all of the discharges. Itis well known that if a large resistance be connected in series with thedischarge path of a gas discharge tube, the resistance may limit thedischarge current to a value insufiicient to produce enough ions tomaintain the discharge. However, if the resistance be shunted by acondenser, a discharge can be initiated, which however very quicklyextinguishes itself when the condenser becomes charged. Such a circuitmay be arranged in various other ways, and is called for convenience asquegging" circuit. The action of the discharge in extinguishing itselfis called a squeg.

The ion recombination time for the gas should be such that the tube issubstantially in its initial unfired condition before the next operatingpulse arrives. The time constant of the elements 5 and 6 should be suchthat the condenser 5 is substantially discharged before the next squegis due to take place.

The discharge which spreads along the cathode 3 may be partiallystabilised by means of the rectifier II which is connected from asuitable tapping point on the battery 8 to the anode 2 through a highfrequency choke coil 12.

When the squeg takes place, a positive potential gating pulse of largeamplitude is generated by the cathode 4. This pulse is-applied through aresistor 13 and blocking condenser 14 to one corner l of a gatingrectifier bridge, the diagonally opposite corner 16 being connected toground through a resistor ll.

The amplitude of the gating pulse is limited by means of the rectifierit which is connected from the junction point of elements l3 and I4 to asuitabl tapping point on the battery 8. The gating pulses afterlimitation appear as shown in' Fig. 2b. The rectifier l8 cuts off anyamplitude variations which might be generated in the tube and whichwould be liable to introduce noise.

The corners I9 and 20 of the bridge are connected by a series circuitincluding a battery 2|, and the secondary windings of a pulsetransformer 22 and of a modulating signal transformer 23. The lattertransformer is shunted by a small condenser 24 which by-passes the pulsefrequency components.

Triggering pulses with sloping crests like those shown at c, Fig. 2 aresupplied to the primary winding of the transformer 22. These pulses areobtained from a local source (not shown) and should have the samerepetition frequency as,

and should be synchronised with, the pulses shown at a, Fig. 2. Thetriggering pulses should be applied in positive sense to corner 20 ofthe bridge. The modulating signal wave, which may be a speech wave, forexample, is supplied to the primary winding of the transformer 23.

The potential of the battery 2| should be such that the rectifiers arenormally all blocked'so that the combined potentials of the triggeringpulses and of the modulating signal ar insufficient to unblock them. Thepositive gating pulse generated by the cathode 4 is adapted to overcomethe effect of the blocking battery 2! so that the rectifiers 25 and 26are freed, thus permitting the two transformers to supply a current toresistance ll through these rectifiers. Positive triggering pulses ofthe form shown at c Fig. 2 thus appear at corner 16 of the bridge andare supplied through a blocking condenser 21 to a trigger device 28 ofany suitable type, adopted to generate at terminal 29 a short pulse, orthe leading edge of one, at a time B corresponding to a level such as A,Fig. 2c. The sloping crests of the triggering pulses will be raised orlowered in accordance with the signal potential applied to transformer23, so that the pulse will be generated by the device 28 earlier orlater, (at C or D, for example), as will be well understood by thoseskilled in the art. The trigger device 28 may be designed to be switchedback to the normal condition by the trailing edge E of the triggeringpulse, in which case it will generate duration modulated pulses like theone shown at d Fig. 2, having movable leading edges and fixed trailingedges. Alternatively, the device 28 may be designed to generate veryshort pulses of fixed duration by returning to normal without anyexternal triggering. In this case the pulses generated will betime-phase modulated.

Alternatively, time phase modulated pulses may be obtained bydifierentiating the leading edges of the pulses d of Fig. 2.

. 4 the tube, the leading and trailing edges of the pulses c of Fig. 2should be arranged to occur slightly later and earlier, respectively,than the leading and trailing edges of the pulses a, as indicated inFig. 2.

The other channel modulators will all be the same as Fig. 1, and may beconnected in parallel to the input conductor 30 of the device 28, sinceall the channel pulses are generated at different times. The phasing ofthe operation of the respective modulators should be so arranged thatgating pulses are generated in turn one at a time.

Fig. 3 shows a demodulator for time-phase modulated pulses. It employsthe same kind of tube as that shown in Fig. 1. Elements I to i6,

. l8, I9, 20, 25 and 26 are the same in both figures,

and have similar functions.

It will be assumed, as before, that the system is a six channel system,so that there will be six demodulators similar to Fig. 3 the inputterminals 9 of which are all connected in parallel. The

operation of these six demodulators will be differently phased, as inthe case of Fig. 1, so that each responds only to the pulses of a givenchan nel.

The modulated pulses shown at a, Fig. 4 are applied to terminal 9, Fig.3, this terminal bein connected to the anode 2 through a decouplingresistor 31 in addition to the blocking condenser l0. These pulses arealso delayed by a short interval in the delay network 32 and are appliedthrough a decoupling resistor 33 and a blocking condenser 34 to theprimary winding of a transformer 35, the secondary winding of which isconnected through a blocking battery 36, similar to 2|, Fig. 1, to thecorners i9 and 20 of the rectifier bridge. These delayed pulses areshown at b, Fig. 4.

The tube I counts the incoming pulses in the same manner as in Fig. l,and generates a gating pulse at the cathode 4 for every sixth incomingpulse. The gating pulses are supplied to the corners l9 and 20 of thebridge through blocking condenser I4 and a transformer 31, the secondarywinding of which is connected in series with the secondary winding oftransformer 35.

Rectangular timing pulses as shown at c, Fig. 4 are applied throughtransformer 38 to the corners l5 and iii of the bridge. When a gatingpulse, shown at d, Fig. 4, appears at the same time as one of thedelayed pulses b, the recti- In order to avoid all risk of introductionof noise arising from variations in the operation of fiers 25 and 26 areunblocked, as before, and an output pulse e, Fig. 4, will be transmittedfrom corner i6 through the primary winding of the output transformer 31,which is shunted by an integrating condenser 40. The leading edge of thepulse e coincides with the movable leading edge of thecorrespondingdelayed pulse b, and the trailing edge coincides with thefixed trailing edge of the corresponding pulse 0. These output pulsesare accordingly duration modulated, and the modulating signal will berecovered from the secondary winding of the transformer 31.

Alternatively, the condenser 40 may be omitted and the output pulses maybe demodulated by passing them through a low pass filter (not shown) inthe usual way.

The delay introduced by the network 32 should be suflicient to ensurethat the leading edge of the delayed pulse b which synchronises with thegating pulse d occurs after the latest time 01. occurrence of theleading edge of this gating pulse, which is likely to vary slightly onaccount '15 of the variations in the action of the tube. In

this way the introduction of noise by the tube is prevented.

What is claimed is:

1. In an electric pulse communication system having receiving equipmentassociated with each of n channels, a channel selector comprising a coldcathode gas filled electric discharge tube having a sequence of ndischarge gaps including an output gap, arranged as a counter tube,means for applying a train of input pulses to the tube in such manner asto fire the gaps in turn, means for deriving from the output gap a pulsecorresponding to each nth input pulse, and means for applying the outputpulses as gating pulses to render operative the receiving equipmentcorresponding to a specified channel.

2. A selector according to claim 1 in which the tube comprises an anode,and co-operating therewith a corrugated cathode having n-1 dischargepoints the first of which is nearer to the anode than the others, and anoutput cathode forming a sequence of n discharge gaps, means forapplying to all the gaps a maintaining potential insuflicient toinitiate a discharge across any of the gaps, means for applying theinput pulses to the anode in such manner as to fire the gaps insuccession one at a time, by successive pulses, means for causing theoutput cathode on firing to squeg and extinguish all the discharges, andmeans for deriving the gating pulses from the output cathode. f

3. A selector according to claim 2 comprising squegging resistancesconnected in series between the source of maintaining potential and theanode and output cathode respectively.

4. A selector according to claim 3 in which the resistance correspondingto the output cathode is shunted by a condenser.

5. A selector according to claim 2, comprising means for limiting theamplitude of the gating pulses. I

6. A selector according to claim 2, comprising means for stabilising thedischarge as it spreads along the sequence of discharge gaps.

7. A modulator for an electric pulse communication system comprising aselector according to claim 1, means for applying a train of saw toothpulses together with a modulating signal wave through a normally blockedrectifier bridge to a generator of phase modulated pulses, and means forapplying the gating pulses periodically to unblock the rectifier bridge.

8. A demodulator for an electric pulse communication system comprising aselector according to claim 1 in which the said input pulses comprise aninterleaved train of time modulated channel pulses, means for delayingthe input pulses, means for applying the delayed pulses together with atrain of regularly repeated timing pulses through a, normally blocked.rectifier bridge to a pulse demodulator circuit, and means for applyingthe gating pulses periodically to unblock the rectifier bridge.

ALEC HARLEY REEVES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,404,920 Overbeck July 30, 19462,443,407 Wales, Jr. June 15, 1948

